The present invention relates to a microphone provided with a vibrational noise reducing means.
In a microphone, particularly, a portable microphone, the vibrational noise generated by vibrations propagated from a unit case of a microphone often poses a problem. The noise is generated because, when the unit case of a microphone is displaced in a certain direction, mass of a vibrating system including a diaphragm tends to stop at an original position.
In the microphone, a microphone unit is stored in a unit case. The microphone unit is roughly divided into a vibration portion and a conversion portion (a fixed portion) which electrically acts on the vibration portion to convert the vibration into an electrical signal. The output of an electrical signal caused by the sound wave of the microphone relies on the relative displacement or relative speed of the vibration portion and the conversion portion. The relative displacement or relative speed of the vibration portion and the conversion portion are generated not only by the sound wave but also by the vibrations propagated from the unit case.
A typical microphone for obtaining a signal output by the relative displacement is a condenser type microphone. On the other hand, a typical microphone for obtaining a signal output by the relative speed is a dynamic type microphone. Incidentally, the vibrational noise rely on the mass for setting a resonance frequency of a vibration system and its elasticity. From a viewpoint of control systems of the microphone, the mass of the vibration portion is in the relationship of mass control greater than resistance control greater than elasticity control.
Therefore, the magnitude of vibrational noise is generally in order of double directivity ribbon (or dynamic) microphone greater than non-directivity dynamic microphone greater than non-directivity condenser microphone. Out of portable microphone, in a single directivity dynamic microphone, particularly, the vibrational noise poses a problem as a handling noise.
The handling noises include the vibrational noise of a low frequency component like a sound xe2x80x9cpon-ponxe2x80x9d generated when the microphone moves so as to pat the thumb of the hand which holes the microphone, and the vibrational noise of a relatively high frequency component like a sound xe2x80x9ckasa-kasaxe2x80x9d generated when the thumb rubs on the microphone. The noise of the low frequency component has the directivity of cos xcex8 with respect to the vibration axis of the diaphragm, but the noise of the high frequency component has not specific directivity since it is generated by the solid propagation of the channel consisting of unit casexe2x86x92elastic support materialxe2x86x92microphone unit.
Means for reducing (preventing) the vibrational noise so far known include a method for mechanically isolating vibrations by the shock mount, and a method for offsetting the vibrational noise by mounting a unit for detecting the vibrational noise in addition to a normal microphone unit.
First, the former shock mount method will be explained. This is a method for isolating vibrations of the microphone unit using a viscoelastic body as gum when the microphone unit is mounted on the unit case, for example, as shown in Japanese Patent Laid-Open No. 1-197000.
On the other hand, in the latter method for offsetting the vibrational noise, for example, as shown in U.S. Pat. No. 2,835,735, there is used, for a displacement proportional type microphone unit, a displacement proportional type vibration detection unit is likewise used, and in the case of a speed proportional type microphone unit, a speed proportional type vibration detection unit is likewise used, whereby signal outputs of both the units are subtracted to reduce the vibrational noise.
The vibration isolation effect in the former shock mount method relies on the resonance frequency and the resonance sharpness of the vibration system, and the resonance frequency is lowered whereby the frequency band having the vibration isolation effect can be widened.
However, when the resonance frequency is lowered, even in the steady state, the microphone unit is displaced from the normal position due to the gravity. If the strong shock is applied to the unit case from outside, the displacement of the microphone becomes extremely large, and the microphone unit collides with the unit case to sometimes generate a big shock sound.
With respect to the resonance sharpness, the higher the resonance sharpness, the larger the vibration isolation effect at the high frequency. However, the vibrational noise at the resonance frequency becomes increased as compared with the case without support of vibration isolation. From the foregoing, when the resonance frequency is lowered or the resonance sharpness is increased, immoderately, the practical trouble is brought forth.
Further, out of the handling noises, the relatively high frequency components generated when the thumb rubs on the unit case relies on the solid propagation as previously mentioned. Therefore, in the case where the sectional area of the shock mount is large, it is not possible to prevent the vibrational noise in the high band.
According to the latter method for offsetting the vibrational noise, signal output levels and phases of the normal microphone unit and the vibration detection unit are adjusted and subtracted whereby the vibrational noise can be reduced extremely satisfactorily. In practice, however, it is difficult to make the signal outputs of both the units the same in the wide frequency band.
That is, even if the microphone unit has the same construction as that of the vibration detection unit, in the diaphragm of the microphone unit, air normally called the additive mass which vibrates the same as the diaphragm is present. On the other hand, the vibration detection unit is surrounded by a cylinder member so as to prevent the sound wave from entering. Even if the cylinder member is vibrated by the sound wave, the diaphragm of the vibration detection unit is to operate in a manner such that the mass equivalently decreases.
Thereby, in the vibration detection unit, the resonance frequency of the diaphragm rises and the signal output level lowers, and a phase difference occurs in the signal output of the microphone unit. In consideration of the above point, in the aforesaid U.S. Pat. No. 2,835,735, the frequency to be cancelled is limited to a low sound band, and in the frequency band in excess of a middle sound band, the shock mount is used to reduce the vibrational noise.
However, naturally, it is necessary that the adjustment of the signal levels output from both the units and the phases thereof are still done very minutely. Further, it cannot be denied that the balance is lost for some factor (for example, a rise in temperature), in which case, the vibrational noise is likely increased. Further, since fundamentally, the vibration detection unit is required in addition to the microphone unit, it cannot be denied that the cost is high, the weight increases, and the device becomes large in size.
In Japanese Patent No. 57-9279 as a separate prior art, as shown in FIG. 8, in housing a diaphragm 2 and a magnetic circuit portion 3 as a conversion portion in a unit case 1 of a microphone, the magnetic circuit portion 3 side is supported on the unit case 1 through an elastic element 4 such as rubber.
That is, the diaphragm 2 and the magnetic circuit portion 3 are vibrated in the same direction with respect to the vibration of the unit case 1 to thereby not to generate a relative speed between the diaphragm 2 and the magnetic circuit portion 3. Since with respect to the sound wave, the mass of the magnetic circuit portion 3 is extremely large relative to the mass of the vibration plat 2, the magnetic circuit portion 3 is not vibrated by the sound wave but only the diaphragm 2 is vibrated. Therefore, the relative speed occurs therebetween and the signal output is obtained by the voice.
In this method, it is required that the resonance frequency of the diaphragm 2 is made the same as that of the magnetic circuit portion 3 elastically supported. However, the method is fundamentally not a method for offsetting signal outputs of both the units as in the aforementioned prior art. Therefore, the adjustment of the signal level or phase is not necessary, and the reduction in vibrational noise in the wide range of frequency can be made.
However, since the end of the diaphragm 2 is still directly secured to the unit case 1, the relative speed occurs between the diaphragm 2 and the magnetic circuit portion 3 by the vibrational noise solid-propagated from the unit case 1. It is difficult to reduce the handling noise such as xe2x80x9ckasa-kasaxe2x80x9d due to the particularly high frequency component.
An object of the present invention is to provide a microphone which is simple in constitution provided with a means for reducing the vibrational noise capable of reducing the vibrational noise in a wide frequency range.
The microphone according to the present invention comprises a diaphragm which is vibrated by a sound wave received, a conversion portion for electrically acting on said diaphragm to convert the vibration into an electric signal, and a first elastic body and a second elastic body arranged on the upper surface and the lower surface of an peripheral edge portion of said diaphragm, said diaphragm being mounted on the upper surface of the peripheral edge portion through said first elastic body, and said conversion portion having one end side placed in contact with said second elastic body arranged on the lower surface of the peripheral edge portion to house said conversion portion into a unit case.
With the constitution as described above, the vibration speed applied to the unit case is transmitted to the second elastic body on the lower surface of the peripheral edge portion of the diaphragm through the first elastic body on the upper surface of the peripheral edge portion of the diaphragm and applied to the conversion portion (the magnetic circuit portion). By limiting a channel in which the vibration speed is solid-propagated, it is possible to reduce the vibrational noise which is generated when the unit case is rubbed.
In this case, preferably, the first and second elastic bodies are in the form of a ring of substantially the same diameter, and projections are formed at least on one surface at predetermined intervals. According to this, since the solid propagation channel of the vibration speed is further limited, the effect of reducing the vibrational noise is further enhanced.
The stiffness of the first and second elastic bodies is set corresponding to the ratio between the mass of a diaphragm and mass of a converter. Thereby, the vibration speed from the unit case is divided according to the stiffness of each elastic body, and the vibration speed applied to the diaphragm is substantially the same as that applied to the conversion portion to suppress the relative speed displacement.
Further, preferably, the other end of the conversion portion is supported on the unit case through a third elastic body. The third elastic body is provided to apply a bias to the first and second elastic bodies, and by suitably setting a mechanical impedance, a phase difference of vibration characteristics of the diaphragm and the conversion portion in a high region caused by the acoustic impedance is reduced. It is necessary to set the stiffness of the third elastic body smaller than the stiffnesses of the first and second elastic bodies.
The present invention can be applied to the single directivity dynamic microphone as well as an electrostatic capacity microphone.
The microphone according to the present invention has the following effects.
The relative vibration speed and the relative vibration displacement on the diaphragm side and the converter side are made substantially the same with respect to the vibrations solid-propagated from the unit case, whereby the vibrational noise can be reduced satisfactorily.
The vibration detection unit and an electric circuit incidental thereto as in the signal output offsetting method are not necessary. A few elastic bodies as constituent parts will suffice to be added to a normal microphone unit, thus providing a microphone at less cost.
Being the construction for reducing the vibrational noise including the acoustic circuit, it is possible to effectively reduce the vibrational noise in the wide frequency range. Further, the vibration speed is divided by the end of the diaphragm whereby the vibrational noise caused by the solid propagation can be reduced.